1. Field of the Invention
The present invention relates in general to an image projector, and more particularly to an optical system for such a projector which is capable of compensating the keystoning, in which keystoning the projected image on a screen appears as a reversed trapezoidal shape, thereby projecting an image without occurrence of the keystoning on the screen disposed at a desired height.
2. Description of the Prior Art
With reference to FIGS. 1 and 2 which show an optical system for a projector according to the prior art, the system comprises a light source 1 for emitting a light, a light condensing part 2 for condensing the light from the light source, a projected object 3 having an image thereon which is projected on a screen, an image projecting lens 4 for projecting the image of the object 3, and a screen 5 on which the image is projected.
Here, a planar display comprising a film and a liquid crystal is conventionally used as the object 3.
As represented in FIG. 1, the known projector has a construction in that the light source 1, the light condensing part 2, the projected object 3, the image projecting lens 4 are arranged on a common optical path, furthermore, the optical path is identical with the center axis of the image projected on the screen 5.
In projecting the image on the screen 5 by using the known projector, when the main light beam Bc transmitting through the centers of the object 3 and the image projecting lens 4 is directed in order to be perpendicular to the surface of the screen 5, that is, the center height of the projector is identical with the center height of the image on the screen 5, the keystoning does not occur. However, when the main light beam Bc is directed in order to be inclined with respect to the surface of the screen 5, that is, the center height of the projector is not identical with the center height of the image on the screen 5, the image is gradually defocused at upper part of the screen 5 so that there occurs the keystoning in that a reversed trapezoidal image reemerges on the screen 5 even though a rectangular-shaped projected object 3 is subject to be projected.
The keystoning due to the discordance of the center height between the projector and the image on the screen causes the quality of the image on the screen to be deteriorated, thereby resulting in a problem of deterioration of the image quality of the projection television system which is provided with the projector.
In an effort for solving the above problem of the occurrence of keystoning, the projector has to be horizontally located in order to identify the center height thereof with that of the image on the screen, thus the height of the screen is obliged to be limited by the installing height of the projector. In accordance, the projector is obliged to be conventionally installed at a relatively high position in order to identify the center height thereof with that of the screen which has to be conventionally positioned at a relatively high position.
In addition, there has been proposed another type of projector which is capable of compensating the keystoning. In this type of projector, the centers of the object 3 and the image projecting lens 4 are eccentrically arranged with respect to the main optical path of the projector as described in FIG. 3. Thus, this type of projector can compensate the keystoning, thereby causing the image projected on the screen 5 to have the shape which is identified with the shape of the projected object 3 and also is uniformly focused overall.
However, this type of projector which is capable of compensating the keystoning has a disadvantage in that the image reemerges as an upward tendency image on the screen, and the height at which the compensation for the keystoning is accomplished is limited by only one height, thereby inducing a considerable difficulty in compensating the keystoning of the image on the screen. Furthermore, a perfect compensation for the keystoning can not be accomplished.
In addition, as a technique in liquid crystal (LC) has been considerably advanced, the liquid crystal is in general used for the projector, that is, the liquid crystal display (LCD) projector which is provided for a projection color television system requiring a high quality of image and a large screen. However, the known LCD projector can not be efficiently used for the projection color television system because the television system has to frequently change the installing position thereof and also requires the same shaped image as that of the original image to reemerge on the screen.
FIG. 4 shows a representative example of the known LCD projector which is provided with three LCD panels and capable of compensating the keystoning of the image. As represented, the LCD projector has a compact construction and performs color light separation and color light composition.
The LCD projector comprises, as represented in FIG. 4, a white light source 10 for emitting a white light, a 45.degree. inclined ultraviolet/infrared rays filter and mirror 11 which is arranged as inclined to an optical path of the light emitted from the source 10 at 45.degree. angle of inclination, a vertical ultraviolet/infrared rays intercepting filter 12 which is vertically arranged on the optical path, a 45.degree. inclined blue light filter 13 which is arranged as inclined to the optical path and capable of reflecting the red and green lights but transmitting the blue light, and a blue light reflection mirror or an inclined total reflection mirror 14 which is arranged on the optical path at a position opposite to the filter and mirror 11 in order to reflect the blue light from the blue light filter 13. Disposed on a reflection optical path from the blue light filter 13 is a 45.degree. inclined green light reflection mirror 15 which reflects the green light but transmits the red light.
In addition, there are sequentially provided, on the reflection optical path started from the total reflection mirror 14, a light condensing lens 16 and a first monochrome LCD element 17, which element 17 is adapted for treating the blue light signal. A blue light filter or a green light reflection mirror 18 is arranged at a front of the first LCD element 17 in order to reflect the green light but transmit the blue light. Sequentially disposed on the reflection optical path started from the green light reflection mirror 15 are a condensing lens 19 and a second monochrome LCD element 20, which element 20 is adapted for treating the green light signal. A condensing lens 21 and a third monochrome LCD element 22 are arranged at a front of the green light reflection mirror 15, which element 22 is adapted for treating the red light signal.
A 45.degree. inclined reflection mirror 23 is arranged at a front of the third monochrome LCD element 22. In addition, disposed at a cross point of the optical path from the total reflection mirror 23 and the optical path from the green light reflection mirror 18 is a red light filter 24 which is capable of transmitting the red light but reflecting the green and blue lights.
The total reflection mirror 23 and the red light filter 24 are arranged on the same optical path as inclined at 45.degree. angle of inclination, respectively.
In addition, an image projecting lens 25 is arranged at a front of the red light filter 24 and projects an image on a screen 26 in order to reemerge thereon.
Here, a color light separating part of the known LCD projector comprises the blue light filter 13, the total reflection mirror 14, and the green light reflection mirror 15, while a color composition part of the LCD projector comprises the total reflection mirror 23, the red light filter 24 and the green light reflection mirror 18.
The operational effect of the known LCD projector having the above-mentioned construction is well known to the skilled in this art so that the detailed description for the operational effect of the known LCD projector will be efficiently omitted hereinafter.
In case of the known LCD projector which is capable of compensating the keystoning, the compensation for the keystoning is accomplished by upward movement of the optical path of the image projecting lens 25 in the same manner as described in the projector having one panel shown in FIGS. 1 to 3, thereby resulting in a disadvantage in that the extent of compensation for the keystoning is limited by the extent of the upward movement of the image projecting lens. In other words, when the arrangement of the known optical system of the LCD projector is accomplished, the extent of the upward movement of the image projecting lens is limited by the magnitude of the projector such as size of the optical system thereof, thus, the extent of the compensation for the keystoning is obliged to be limited to a narrow extent.